This proposal is for continued support of my ongoing studies on the immunobiology of tumors. My goal is to learn more about immune regulation concerning tumor specific antigens, and I will do this primarily by studying immunity to 3-methylcholanthrene (MCA) induced mouse sarcomas, concentrating on tumors of strong immunogenicity, but also studying such of low immunogenicity. Much effort will go into work on specific blocking factors, suppressor cells and anti-idiotypic antibodies. I have the following five specific aims: first, investigations on the nature of specific blocking factors (SBF) in mice with progressively growing tumors; second, searches for idiotypes on lymphocytes with specificity for tumor antigens; third, suppressor cell activity in tumor-bearing animals, as measured by its effect on primary in vitro sensitization to third party antigens; fourth, tumor enhancement in vivo, including the role and nature of suppressor T cells; fifth, attempts to develop ways for immunological prevention and therapy of tumors based on the above analysis of tumor immunity.